The present invention relates to a lithograph for producing digital holograms in a storage medium. In particular, the lithograph has a light source for producing a write beam with a predefined beam cross section, a writing lens for focusing the write beam onto the storage medium to be written, the writing lens being arranged in a lens holder, and having drive means for the two-dimensional movement of the write beam relative to the storage medium. Furthermore, the invention relates to a method of producing digital holograms in a storage medium.
Digital holograms are two-dimensional holograms which consist of individual points with different optical properties and from which, when illuminated with a coherent electromagnetic wave, in particular a light wave, images and/or data are reproduced by means of diffraction in transmission or reflection. The different optical properties-of the individual points can be reflective properties, for example as a result of surface topography, varying optical path lengths in the material of the storage medium (refractive indices) or color values of the material.
The optical properties of the individual points are calculated by a computer, and this thus involves what are known as computer-generated holograms (CGH). With the aid of the focused write beam, during the writing of the hologram the individual points of the hologram are written into the material, the focus being located in the region of the surface or in the material of the storage medium. In the region of the focus, focusing has the effect of a small area of action on the material of the storage medium, so that a large number of points of the hologram can be written on a small area. The optical property of the respectively written point in this case depends on the intensity of the write beam. For this purpose, the write beam is scanned in two dimensions over the surface of the storage medium with varying intensity. The modulation of the intensity of the write beam is in-this case carried out either via internal modulation of the light source, for example a laser diode, or via external modulation of a write beam outside the light source, for example with the aid of optoelectronic elements. Furthermore, the light source can be formed as a pulsed laser whose pulse lengths can be controlled, so that control of the intensity of the write beam can be carried out via the pulse lengths.
As a result of the scanning of the intensity-modulated write beam, an area with an irregular point distribution is thus produced, the digital hologram. This can be used to identify and individualize any desired objects.
Scanning lithographic systems are intrinsically widespread. For example, scanning optical systems are incorporated in conventional laser printers. However, these systems cannot be used for the-production of holograms, since the requirements for this intended application differ considerably from those in laser printers. In the case of good printing systems, the resolution is around 2500 dpi while, in the production of holograms, a resolution of about 25 000 dpi is required. In addition, in digital holography, only comparatively small areas are written. These are, for example, 1 to 5 mm2, other sizes also being possible. The accuracy of the write pattern in the case of a lithograph for the production of digital holograms of, for example, 1000×1000 points on an area-of 1×1 mm2 must be about ±0.1 μm in both orthogonal directions. Furthermore, the writing speed should be about 1 Mpixel/s, in order that in each case a hologram can be written in a time of about 1 s. The aforementioned magnitudes are exemplary and do not constitute any restriction of the invention.
Digital holograms can be produced by means of conventional scanning methods, in which the angle of the incident beam is varied by stationary optics. For example, scanning mirror lithographs with galvanometer and polygonal scanners operate on this principle. However, scanners of this type have the disadvantage that the implementation of this principle entails a great deal of optical and mechanical effort. This fact places tight limits on the maximization of the speed and the resolution of optical lithographs, since, for this purpose, objectives are needed which permit a large field angle and convert the deflection angle, preferably linearly, into an x deflection in the focal plane of the objective (“F-theta” objectives). Moreover, the objectives used have to be corrected with regard to the image curvature (“flat field” objectives), so that complicated multi-part optics have to be used which are an obstacle to a compact configuration of the lithograph. Furthermore, complex optics of this type place great demands on the mechanics of the lithograph, since the latter have to move a relatively large mass. This also results from the fact that it is not possible to select arbitrarily small scanning mirrors, since the aperture of the optical system always determines the resolution as well.
However, scanning optical systems are also known in which the scanning movement is not achieved via a moving beam but via moving optics. However, the scanning speeds and accuracies of the positioning of the write beam which permit a predefined point pattern of the digital hologram to be maintained for the writing speeds to be achieved are not achieved here either.